The present invention relates generally to the manufacture of cast concrete structural members and, more particularly, to an improved apparatus for manufacturing such concrete members in a plurality of different widths.
To efficiently manufacture cast concrete members in the form of slabs to be used, for example, as walls, floors, or roofs in building construction, a batch-type process has been employed. In a batch-type process, a long concrete slab or plank is poured and cured, and thereafter cut into a plurality of shorter length sections. Due to the relatively long curing time required (approximately 14 hours), this batch-type process is more efficient than casting individual shorter slabs because a large number of individual planks may be produced at substantially the same time. The length of the slab before cutting into shorter lengths may be as much as 150 meters (500 feet), with the slab weighing approximately 230,000 kg. (500,000 pounds). The shorter lengths into which the slab is cut may range from approximately 2.4 meters (eight feet) long for a wall span, to 15.2 meters (fifty feet) long for a roof span. Of course, other lengths are possible, as desired.
In the above-described process, the concrete slab is prestressed by means of reinforcing strands or cables longitudinally tensioned between the ends of the casting bed prior to the casting of the concrete. After the concrete is cured, the ends of the reinforcing strands are released to transfer the stress to the concrete.
One limitation in both the manufacture and the use of precast concrete slabs relates to the capacity of cranes used to handle the slabs. Cranes are employed both at the manufacturing facility to lift the completed slabs from the casting bed onto suitable storage or transportation means, and at a construction site to lift the slabs into proper position on the building structure. For efficient use of the casting apparatus, it is desirable to cast the slabs in as wide a width as practical, for example 2.4 meters (eight feet). However, a slab 2.4 meters (eight feet) wide by 15.2 meters (fifty feet) long, such as might be employed for a roof span, may be too heavy for cranes available at a construction site to lift, while a shorter length, for example an 2.4 meter (eight foot) wide by 4.5 meter (fifteen feet) to 6.1 meter (twenty foot) long wall span, may be within the lifting capacity of the crane. Accordingly, in situations where crane capacity is limited, it is desirable to manufacture slabs of lesser width, for example 1.2 meter (four feet), where the longer lengths are being manufactured. Overall, the most efficient operation results where the widest possible slabs are manufactured for each circumstance.
It is therefore desirable to be able to manufacture such slabs in a plurality of widths, such that as the lengths of the completed slabs are varied the overall weight may be kept within the crane capacity. For example, 2.4 meter (eight foot) wide slabs may be manufactured for the shorter lengths, and 1.2 meter (four foot) wide slabs for the longer lengths. A previously employed approach to this problem has been to cast all slabs in 2.4 meter (eight foot) widths, and then saw longitudinally. While this accomplishes the desired result, it has the disadvantages of increased usage of expensive diamond toothed saw blades, and requires additional costly labor time.
By the present invention there is provided an improved apparatus for manufacturing elongated concrete structural members in a plurality of widths. Apparatus embodying the present invention is particularly adapted for use in making concrete structural members employing either a fixed bed apparatus of the general type described in the commonly-assigned U.S. Pat. No. 3,217,375 of Kinnard, or a moving bed apparatus of the type described in the commonly-assigned U.S. Pat. No. 3,523,343 of Mitchell.
In the Kinnard U.S. Pat. No. 3,217,375, the concrete structural member, including core openings therein, is produced on a stationary casting bed by means of hopper and roller units and a casting machine which travel the length of the bed. The completed member is then cured and subsequently cut into desired lengths.
The Mitchell U.S. Pat. No. 3,523,343 describes a moving bed system wherein a manufacturing area contains sequentially used items of production equipment suitably housed and located centrally between an elongated curing area and a similarly elongated unloading overrun area. A casting bed moves back and forth several times during each production cycle while the various operations of the manufacturing process are performed. The casting bed comprises a lengthy stress frame which is mounted on a track extending the full length of the facility and carries a molding form in which the concrete member is cast by equipment which normally remains stationary in the manufacturing area while the bed moves beneath it. The casting bed is moved along a track at any desired speed in either direction by suitable driving means such as two sets of selectively operable hydraulic motors which are located on opposite sides of the manufacturing area and which drive rubber tired wheels having frictional engagment with the stress frame.